???40mm seems quite a lot, have you adjusted the legs on the appliance?
When appliance is fitted to right height, the underside of the appliance will then screw in to the worktop.
How would you get into a washer to screw it to the work top when it is in situ? Or if you mean jacking it up on the height adjusters, wouldn't that make leveling difficult & transmit vibrations to work top & thus the surrounding cupboards etc.. Not being critical, just never seen that done before.
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